tests: Fix level test in CK_FORK=no mode
tests: Fix level test in CK_FORK=no mode

The tests accumulate buffers in GstCheck's buffers list, and the list is
not (consistently) reset between tests. Do that and remove the now
conflicting unrefs for outbuffers.

https://bugzilla.gnome.org/show_bug.cgi?id=772644

GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good